Parametric amplification is the amplification of light (the "signal" and "idler" light energies) at two frequencies at the expense of light (the "pump" light) at another frequency. Parametric amplification is generally accomplished by directing a beam of the pump light onto a nonlinear medium. Two types of optical cavities have been used in practice to increase the gain obtainable from the nonlinear medium by parametric amplification. These resonator types are termed singly and doubly resonant oscillators (SRO and DRO, respectively). By maintaining a strong feedback at one or more of the frequencies being generated by the parametric process, the conversion efficiency of the pump beam energy into energy at the two generated waves can be increased at energy pump intensities significantly lower than are required for simple optical parametric generation where no cavity structure is used.
The addition of an optical cavity reduces the effective gain bandwidth of the nonlinear medium to frequencies that are resonant with the mode or modes of the cavity. The doubly resonant oscillator typically displays instabilities in operation, since it is overconstrained by the phase restrictions introduced by requiring that the cavity be resonant at the frequencies of both the signal and idler energies.
The instabilities of the doubly resonant oscillator are overcome in the singly resonant oscillator by using a cavity that is resonant at only one of the generated signal and idler frequencies. In the singly resonant oscillator, the frequency of the energy at one frequency is fixed by the resonant cavity while the energy at the other frequency is not resonated and is free to compensate for phase and frequency changes in the pump source.
While the singly resonant oscillator does not have the instability inherent in the doubly resonant oscillator, it does have the drawback that its performance is optimized for only one of the two generated waves. Intracavity losses and the increased photon residence time in the cavity at the resonated wave are responsible for this effect.
One major drawback presented by both the singly and doubly resonant oscillators is that their performances are fairly sharply peaked functions of incident pump intensity. At pump intensities below this peak, their conversion efficiencies increase rapidly with increasing pump intensity due to the feedback offered by the resonant cavity. As the pump intensity is increased above this peak, this feedback increasingly serves to drive the process in reverse and the efficiency of conversion decreases. This sensitivity to variation in incident pump intensity contrains the conditions under which single and doubly resonant oscillators can be efficiently operated.